Refers to the strength of the relationship between two variables, for example the relationship between: weather and items of food or clothing – for example – in winter, sales of umbrellas are likely to increase and sales of ice creams are likely to decrease; expenditure on promotion and sales – for example – an increase in advertising expenditure is likely to result in an increase in sales; pay rates and productivity – for example – a rise in pay rates may result in an increase in labour productivity; complimentary goods / services – for example – the greater the demand for razors, the greater the demand for shaving foam; Substitute goods / services – for example – the greater the demand for houses, the lower the demand for rented accommodation. Establishing a relationship between different variables might assist a business with the prediction of sales and, in particular, in making decisions about marketing strategy and the various elements of the marketing mix, as well as decisions relating to other functional areas (eg HR and levels of pay).